Motorola MC68VZ328 17.1.1 Entering Bootstrap Mode, 17.1.2 Bootstrap Record Format

17-2 MC68VZ328 User’s Manual 

Bootstrap Mode Operation

17.1.1 Entering Bootstrap Mode

Bootstrap mode is one of the three operation modes (normal, emulation, and bootstrap) of the

MC68VZ328. Of the three modes, bootstrap has the highest priority. To enter bootstrap mode, the

EMUBRK signal must be driven low and a system reset must be performed. After reset, bootstrap reset

vectors are internally generated for reset vector fetch cycles. Figure17-1 illustrates bootstrap mode reset

vector fetch timing. These two-long-word reset vectors are loaded to the st ack point er and pro gram coun ter

of the CPU, and then the built-in bootstrap program runs and accepts data transfers.

Figure 17-1. Bootstrap Mode Reset Timing

17.1.2 Bootstrap Record Format

Bootstrap mode data transfers will only accept bootstrap records (b-records) whose format is shown in

Table 17-1. The two major attributes of b-records are that they are in uppercase and end with a carriage

return.

There are two types of b-records that use the same format. The data b-record contains data to be

transferred. The 4-byte address field indicates where the data will be stored, and this address could be any

MC68VZ328 internal register location. The count field of the record contains the number of data bytes to

be transferred. The data field contains the data to be transferred.

The execution b-record tells the bootloader to run a program starting at the location specified by the

address field of the b-record. The count field for an execution b-record always contains 0x00, and no data

is in the data field.

An execution b-record is used in two situations:

• After a program is downloaded to system RAM, issuing an execution b-record initiates program

execution. In this case, the address field of the b-record will be the start address of the program.

• When loading a 68000 instruction into the instruction buffer and filling th e remainder of the unu sed

buffer space with nop--$4e71, issuing an execution b-record executes the 68000 instruction that

is stored in IBUFF and returns to bootloader mode. In this case, the address field of the b-record

will be the start address of IBUFF.

Table 17-1. Bootstrap Record Format

4-Byte 1-Byte N(Count)-Byte

Address Count Data

addr

data

EMUBRK

RESET

0000 0002 0004 0006 ffffff00 ffffff02

ffff ff00fffcfffc